Papillary muscles were attached to an isometric myograph in the presence of Pi32. Treatment of contracting papillary muscles with norepinephrine results in a 2-fold increase in phosphorylation of myosin light chains and 4-fold increase in phosphorylation of M-line protein compared to controls. The 165,000 dalton M-line protein from dog and human hearts has been isolated and phosphorylated in vitro.